Lu Zikuan’s dad, who was diagnosed seven years ago, needs a bone marrow transplant.
Zikuan, from Xinxiang in China, is the only match within his family, but needs to put on weight before the procedure can go ahead.
Doctors say he needs to weigh at least 45kg (seven stone) before he can become a donor, and the ideal weight is 50kg.
When the family was told this, he was only 30kg.
His father, Lu Yanheng, was being treated with medication until his health deteriorated last August and he needed a blood transfusion.
His only long-term option is a bone marrow transplant, doctors said.
Zikuan has been eating mainly fatty meat with rice, and now weighs 43kg.
His mother relies on discounted meat from the grocery store she works at.
As Zikuan began to put on weight, some of his fellow classmates called him pangzi (fat), his teacher Zhao Meng-meng said.
But they stopped after discovering why he was getting bigger.
Zikuan said he was not angry about being called fat. “Save dad first, lose weight later,” he told local media.
The school is fundraising to help his family.
People on social media have been moved by his story. Kongwei Wizard said he was shouldering a “great responsibility at such a young age”, adding: “You are amazing.”